The present invention relates to a process for forming MOS programmable memory elements using metal fuse links.
Devices used to provide programmable memory elements in MOS processes have included the electrically programmable read only memory cell (EPROM) or the erasable electrically programmable read only memory cell (EEPROM). Such non-volatile memory elements are generally costly to fabricate. For example, the EPROM requires each chip to be erased at least two separate times during fabrication. This involves a large amount of process time. An additional problem with these devices aside from their relatively complex processing resides in the amount of chip area each of them occupies. With present day technology each EPROM takes up at least about 20 square microns and an EEPROM at least 200 or more square microns depending on cell design.
Fuse links have been used in MOS design but these have been of polysilicon. Polysilicon fuses require a hole in the passivation layer so that the gases released upon "blowing" of the fuse can escape. Such openings expose the chip to potential corrosion through the opening. A second problem with polysilicon fuses is that they are not compatible with plastic packages and to date have only been built with reliable yields in the relatively costly ceramic package.
Metal fuse links have been used in bipolar processes but such processes involve different oxides and larger layouts and are capable of absorbing larger currents than MOS devices. It has not been necessary therefore to attempt to design a fuse which "blows" with currents less than about 25 milliamperes as would be required by MOS devices. Even if lower fusing currents are possible it is not known whether such fuses could be manufactured without the objectionable opening in the passivation layer necessary for polysilicon fuses. Another problem with using fuse links in MOS processes resides in the fact that MOS circuits are significantly more sensitive to contamination such as that from etching of the fuse link metal. Finally, it has not been known whether deposition techniques for suitable fuse metals used in bipolar processes would be applicable to MOS processes.
Accordingly, it is an object of the present invention to provide an improved process for producing programmable memories in MOS processes. It is a further object of the invention to provide a process for making a metal fuse link in an MOS process.